One of my A500's does not work well with a ScanDex external scandoubler. The image flickers on and off like someone is plugging in and unplugging the cable. It comes and goes but mostly stays black. I opened up the machine and made sure the pins on the RGB port are making a connection to the MB--they do. I even touched each pin and melted the solder to make sure there was no cold solder joints, but that is far as my tech knowledge/abilities goes. It also works fine on a RGB monitor. It has a Rev 6A MB, 1MB Agnus, 1MB Chip, and KS 1.3. Another A500 I have works fine on the scandoubler. It has a 512K Agnus, 512k Chip and KS 1.3 as well. Not sure what MB it has though. Any ideas?